1. Field of the invention
An apparatus for measuring the density of a contained fluid medium by utilizing the angular displacement of the limiting angle of rays with a light transmitting rod.
2. Description of the prior art
The prior art is represented by U.S. Pat. Nos. 2,483,102 to R. M. Pierson of Sept. 27, 1949 for "Refractometer Employing Photosensitive Devices and Use of the Same", and 2,569,127 to G. C. Eltenton of Sept. 25, 1951 for "Refractive Index Measurement of Fluids".
For measuring the density of a liquid, an apparatus with a glass rod immersed in the liquid to be measured is already known. In such a structure, a light source is provided at the end of the glass rod that projects from the liquid, which produces a pencil of light rays which enters parallel to the optical axis of the glass rod. At the end of the immersed glass rod, there are two deflecting surfaces and a measuring surface. The deflecting and measuring surfaces are arranged in such a way that the light pencil passes, by way of one of the deflecting surfaces, at the angle of total reflection, to the measuring surface and therefrom, by way of the other deflecting surface, to an optical system.
Such an apparatus presents the disadvantage that the density of the liquid can be determined only within very narrow limits and subjectively. A continuous measurement of the density by means of this apparatus is therefore not possible.
In another known apparatus, a photoconductive rod is provided. In this structure, at the end of the rod that is immersed in the liquid, a front surface is provided which is perpendicular to the rod axis and which is provided with a reflecting coat. At the rod end that projects from the liquid, a light source and a photosensitive element are provided. In such an apparatus, the light rays entering the photoconductive rod are refracted into the liquid to a greater or smaller extent, depending on the density of the liquid, so that the quantity of light that passes into the liquid is a criterion for the density of the liquid. Such an apparatus makes a continuous measurement possible. However, care must be taken that the depth of immersion of the photoconductive rod always remains the same, since otherwise substantial errors of measurement may result. However, especially in storage batteries, the liquid level varies considerably, so that such an apparatus cannot be used for measuring the acid density of a storage battery.
Another apparatus for measuring the acid density of a storage battery is known, in which a pencil of rays departing from a light source is directed toward several photosensitive receivers through a prism filled with the liquid. Depending on the density of the liquid, the pencil of rays is refracted to a greater or smaller extent by the prism and therefore strikes, according to the index of refraction prevailing in each instance, a photosensitive receiver associated with that index of refraction. Such an apparatus presents the disadvantage that it is of a relatively large structural volume and cannot be installed in the battery casing without extensive alterations thereon. Moreover, this type of an apparatus consists of expensive structural parts and is therefore itself correspondingly expensive.
Finally, an apparatus for measuring the density of a liquid has been proposed which comprises a photoconductive rod with a measuring surface and two reflecting deflecting surfaces at the end of the rod immersed in the liquid, comprising a light source radiating a parallel pencil of rays, and a photosensitive element with a filter wedge arranged in the path of the ray in front of the said photosensitive element. The pencil of rays is directed toward the measuring surface by way of one of the two deflecting surfaces, at an angle with the right angle of the measuring surface, which equals at most the total reflection angle occurring at the lower liquid density limit, and the ray pencil refracted by the surface into the liquid is directed toward a boundary surface of the rod perpendicular to the axis of the ray pencil, behind which boundary surface the other deflecting surface is positioned. By means of such an apparatus it is possible to measure continuously the density of the liquid independently of the liquid level prevailing at each instance. In addition, the structure in comparison with the known devices is cheaper and consists of fewer structural parts, and presents a smaller structural volume so as to be particularly suitable for measuring the acid density of a storage battery.